The printed circuit board (PCB) is the most used component in the world today. It is found in every electronic device, and it doesn’t matter where you live, PCB’s are used by everyone all the time. It is safe to state, that ever since the PCB was first invented, it has become the most important component in human life.
PCB’s are basically layouts of circuits that connects a power source to a circuit that performs a function. There are three circuit board designs that are used for most of all the applications, and these include:
Single-Sided Circuit Boards
Single sided CB’s are made of an FR4 base and have a rigid laminate of woven glass epoxy material; this is covered on one side with a copper coating.
Double-Sided Circuit Boards
Double-sided PCB’s use the same woven glass epoxy base but there is a copper coating on both sides of the board.
Multi-Layer Boards
Multi-layer PCB’s use the same laminate, but have a copper foil to make alternating layers between the base material and copper foil for the desired number of layers.
A PCB is made up of 4 parts
- Substrate: Rigid PCB’s use Fiberglass, Flexible PCB’s use PET.
- Copper Layer: This is either copper foil or a copper coating.
- Solder Mask: This is a layer of polymer used to protect the copper.
- Silkscreen: This is the “map” an outline of the components, lines and Logo’s that will be placed on the board.
Fabrication Process
- PCB Design
Special software is used to design the entire PCB; this includes the number of copper layers, the amount of solder mask required and the other component notation items. The design is sent to the PCB fabrication plant where it undergoes a Design for Manufacture (DFM) check.
- Printing
A transparent negative film of the board is prepared on a plotter. The circuit design of the inner layer uses black ink to show the circuitry, and the empty or clear board areas are transparent. The outer layer is the opposite, where the copper layer is denoted in transparency, and the clear areas (copper free) are black.
Each layer of the PCB has a film, so for a two-layer PCB, there will be four films.
Registration Hole: After the films are printed, a hole is punched through them which is used for alignment during the fabrication process.
- Printing the Interior Layer Copper
The laminate layer is covered with the copper film, and the black areas are cleared of copper (etched) leaving only the map of copper on the surface. This surface is covered with a laminate panel called “resist” which is made of a photoreactive chemical that hardens under UV light. The exposed areas harden, and any black areas remain soft for removal later in the process.
The board is washed in an alkaline solution that removes any residual resist. This leaves only a layer of resist over the copper outline.
- Removing unwanted Copper
A special solution is used to etch away any unwanted copper. The copper that is not covered by resist will be eaten away.
- Layer Alignment (QA)
Special optical equipment is used to align all the layers of the PCB. Using the registration hole as the base for alignment, an optical punch is used to drive a pin through the holes to fix the layers together. After this, the PCB is inspected for any errors.
- Lamination
The PCBs are aligned and laminated, this process starts with the lay-up step and followed by the lamination step.
The outer layer is covered with a thin layer of epoxy resin, then a layer of the substrate is covered with this resin, and then a layer of copper foil, followed by another set, for all the layers required. The last layer is a “press plate” made of copper. These layers are pressed together in heated plates, causing the resin to melt, bond the layers, and harden after cooling. The pins are also pulled free during this step.
- Drilling
A special drill is used to drill the holes in the design, and any copper is cleaned from the edges.
- PCB Plating
This is a chemical process that infuses all the layers, and the top panel and holes are coated with a micron-thick layer of copper.
- Tin Plating the Outer Layer
The outer layer is covered in a thin layer of tin that is used to protect the outer layer of copper.
- Final Etching
All unwanted copper is cleansed away with the final etching process.
- Solder Mask Application
The PCB panels are covered in epoxy, and this is covered by the solder mask film. The PCB is exposed to UV light that hardens these together. Any unwanted solder mask is removed, and then the PCB is placed into an oven for final hardening.
- Pad Application
In this step, the hardened and nearly finished PCB pads are given a coat of gold, silver or HASL for easy soldering of components.
- Silkscreen Application
The finished PCBV is now printed with all the vital data including manufacturer marks, ID numbers, and warning labels.
- Cutting and Profiling
PCB’s must meet certain physical dimensions, and these are reached by cutting the final design into the required shape. This is performed with a CNC machine or a V-Groove.
Note: During the fabrication process, QA is performed at every stage, and electrical conductivity testing is performed before silkscreen to assure product integrity.
